In recent years, there has been much interest in multiple input, multiple output (MIMO) systems for enhancing data rates in mobile communication systems. MIMO systems employ multiple antennas at the transmitter and receiver to transmit and receive information. The receiver can exploit the spatial dimensions of the signal at the receiver to achieve higher spectral efficiency and higher data rates without increasing bandwidth.
One transmission scheme for MIMO systems that is receiving significant attention is spatial multiplexing. In a spatial multiplexing transmitter, the information symbols are precoded before transmission to multiplex the information signal in the spatial domain. The precoding may be channel dependent or channel independent. With channel dependent precoding, also referred to as closed loop precoding, the precoder matrix is chosen to match the characteristics of the MIMO channel. With channel independent precoding, also referred to as open-loop precoding, channel characteristics are not considered in selecting the precoder matrix.
With closed loop precoding, the user equipment performs channel measurements on the forward link channel, and feeds back channel information or precoder configurations to the base station. One problem with closed loop precoding is that it takes time to perform channel measurements and feed back information to the base station. During that time, the channel conditions may have changed so that the feedback information is outdated before it is used. Consequently, closed loop precoding is typically used in low mobility situations where the channel variations are slow.
In situations where the channel conditions vary more rapidly and lack significant long-term properties, channel independent precoding or open-loop precoding may be used. With open loop precoding, the precoding matrix is selected independent of the channel realizations. Channel independent precoding is generally considered more suitable for high mobility situations.
One way to implement open loop precoding is to use a spatial multiplexing precoder matrix to precode the information sequence prior to transmission. In order to accommodate a wide range of channel realizations, it is advantageous to apply multiple precoders that are varied in a deterministic manner known to both the transmitter and the receiver. For example, in an orthogonal frequency division multiplexing (OFDM) system, the precoder may be kept fixed for a set of one or more subcarriers and then changed for the next set of subcarriers. This technique, referred to as precoder cycling, serves to distribute the energy spatially in a more isotropic manner, which in turn is useful for diversity and reducing the tendency to bias the performance of the transmitter for a particular set of channel realizations. When applying precoder cycling, it is advantageous to have substantial precoding variation over the smallest possible allocation unit, e.g., a resource block (RB), since a codeword may potentially only span a very small set of resource elements.
A number of drawbacks have been encountered in the past when precoder cycling has been used. Interference rejection algorithms implemented at the receiver need to characterize the spatial properties of the channel to suppress interference. It is beneficial that the interfering transmissions have roughly similar properties over a large number of resource elements so that averaging may be used to suppress noise and other impairments. In systems where the cycling of the precoder is configurable, the receiver can not be sure how fast the interference changes over a resource block without a priori knowledge of the precoder sequence. Also, the precoders are frequently chosen from a codebook designed for channel dependent precoding. As a consequence, the precoders do not distribute the energy uniformly over the vector space of the MIMO channel. Finally, precoder cycling increases the computational complexity of demodulation and CQI computation. The computational complexity is bounded only by the codebook size, so the receiver needs to be designed to handle the worst case scenario.